Before proceeding, you should make a backup of your current bookmarks. You can do this by opening the Bookmarks Library window (explained below), then selecting Import and Backup and choosing Backup…. This will create a Firefox bookmark backup file in JSON format.

Note: The imported bookmarks will be added to your existing bookmarks, which may result in duplicate bookmarks. You can delete unwanted Firefox bookmarks before importing a new set of bookmarks, but this can be time-consuming. If you wish, you can delete all of your bookmarks at once from Firefox Safe Mode by selecting the Delete all bookmarks except for backups option and restarting Firefox.

Click the Bookmarks button
and select Show All Bookmarks to open the Library window.

From the toolbar on the Library window, click Import and Backup and choose Import Bookmarks from HTML....

In the Import Bookmarks File window that opens, navigate to the bookmarks HTML file you are importing and select the file.

Click the Open button. The Import Bookmarks File window will close.

Close the Library window.

The bookmarks in the selected HTML file will be added to your Firefox bookmarks within the Bookmarks Menu directory.